structure and evolution of stars - university of cambridgephewett/sandes2017/lec2017... · 2017....

Structure & Evolution of Stars 1

Structure and Evolution of StarsLecture 20: Supernovae, Core Collapse and

Remnant Properties (#1)•Maximum ZAMS mass for formation of a White Dwarf• Observations of Supernovae – supernovae types from spectra• Core Collapse Supernovae

- Photodisintegration- Electron Capture or Neutronisation- Supernovae Energetics- Neutrino Emission and SN1987A- Estimate mean-free-path of neutrinos at nuclear densities


Core Collapse Supernovae and Exotic Remnants• White dwarfs are extreme in terms of their densities but the physical properties are relatively well understood.• Stars with ZAMS masses in range 0.1-10Msun believed to result in white dwarf remnants, Helium composition for low-mass stars with no Helium ignition on giant branch and Carbon+Oxygen composition for stars that ignite Helium on the giant branch• The upper mass limit is not well determined but is believed to lie in the range ~8-11Msun • How can the mass limit be more tightly constrained via observations?• From the upper end, new studies of supernovae progenitors, as in Star of the Week #2, now making progress• From the lower end, use observations of young stellar clusters, looking for presence of white dwarfs as a function of main sequence turnoff mass


HR-diagrams for 3 stellar clusters of very different ages

Radically different morphology of HR-diagrams as a function of age. An “isochrone” describes location of stars of fixed age in HR-diagram. (M3 ≈ 11 × 109yr; Hyades ≈ 6 × 108yr)


• SN2001B

Type Ib – no Hydrogen lines but believed to result from core collapse

What type of star might produce a spectrum with little Hydrogen evident?

16

Different supernovae as a function of initial stellar mass and the metallicity.

Assumes that a supernova is produced even when a blackhole formed


Core Collapse Supernovae and Exotic Remnants

• Above the maximum ZAMS mass for creation of a white dwarf remnant, stars run through the full nuclear burning spectrum up to the peak at Iron in the binding energy per nucleon• Core of inert Fe grows as Silicon burning in shell deposits more Fe onto the core• Lack of energy source within the core leads to contraction, with electron degeneracy pressure providing the resistance to gravity• As mass of Fe core approaches the Chandrasekhar mass (about 1.4Msun for composition of Fe) electron velocities become relativistic. Equation of state tending towards

and core becomes unstable and will collapse on dynamical timescale

3/4ρ∝P

s103

1kgm10Density 23-12 −≈≈⇒≈ρ

τρGdyn


Core Collapse: Photodisintegration• Stability of the core is undermined by photodisintegration of the nuclei• As the density increases the temperature of the core rises and the photon energies reach point where photodisintegration occurs:

n4He13Fe 42

5626 +→+γ

• Reaction absorbs energy, 124.4MeV per Fe nucleus. 1kg of Fe into He requires 2×1014J, equivalent 50kilotons of TNT!• At even higher temperature Helium photodisintegration occurs:

n2HHHe 11

11

42 ++→+γ

• For a 1.4Msun core, 4×1044J (Fe→He), 1 ×1045J (He→H), equivalent to the output of the Sun over 1010yr. Photodisintegration effectively reverses nucleosynthesis!


Core Collapse: Electron Capture• Under normal circumstances, neutrons are unstable, with a short half-life of 615s, the decay resulting in a proton, an electron and an anti-neutrino:

ν++→ en H11

• Energy of electron =1.3MeV but in the fully degenerate core all the available energy states for the electrons are full (Lecture 8) well above 1.3MeV and the decay cannot occur• Large electron energies mean that the reverse process can occur with 3.7MeV electrons:

ν

ν

+→+

+→+

MnFe also

H

5625

5626

11

e

ne

• This neutronisation continues with subsequent e-capture to give Cr


Core Collapse: Electron Capture

• The effect of the neutronisation is to remove electrons from the core, causing the core to collapse as resistance to gravity reduced• Electron capture reactions also produce neutrinos, which have very low cross-section for interaction with matter even at the core densities• Neutrinos escape from the core, free-streaming for ρ<1014kgm-3, removing more energy, exacerbating the predicament of the core• An Fe-core with the Chandrasekhar-mass contains ~1057

electrons, each of which leads to creation of a neutrino during electron capture. With an average energy of 10MeV per neutrino, the energy lost from the core is:

J106.1)106.110(10 451357 ×=×××≈ −captE

• Core collapses on timescale of 10-3s


Core Collapse: Energetics

• Collapse continues until halted by the neutrons packed sufficiently close that the object has nuclear density, ρ≈1018kgm-3, with pressure deriving from the degeneracy pressure of the neutrons (c.f. electrons for white dwarfs)

J102MeV7

J103

11

45

462

2

×≈≈∆

×≈≈

−−≈∆

H

cnuc

ns

c

nsccgrav

mME

RGM

RRGME• Enormous amount of gravitational

PE liberated during collapse:

• Compare to amount required to reverse the fusion of H to Fe:


Core Collapse: Energetics

J10)(5.0

J105)(

J103

yr1 J;10L103

452exp

44

44

37sun

10

≈−≈∆

×≈−

≈∆

×≈≈∆

≈≈×≈

vMME

RMMGME

LE

L

ckinetic

c

ccbind

SNSNrad

SNSN

τ

τ• Manifestation of collapse is a supernova with observed luminosity for characteristic time:

• Energy required to unbind the stellar envelope:

• Energy required to accelerate envelope to expansion velocities of 10000kms-1:


Core Collapse: Energetics• Combined energy required to explain all manifestations of core collapse in a supernova amounts to <10% of the energy available from the collapse of the core to neutron star dimensions• Prediction of the model is that the bulk of the energy leaves the core in the form of neutrinos (neutrino produced for every elecron and proton combining to produce a neutron via a weak interaction)• Predicted number of neutrinos at the Earth due to the core collapse that produced SN1987A ~1013m-2!• Neutrino detectors were operational in South Dakota and Japan• US detector sits 1500m below ground, consisting of tank of 600 tons of cleaning fluid (C2Cl4) in an abandoned gold mine• Small probability a rare isotope of Chlorine absorbs a neutrino to produce a rare isotope of Argon, the presence of which can be measured via radioactive decay• Total of 20 neutrinos detected in two detectors!


SN1987A: Neutrino Detection• Energies of the neutrinos detected are consistent with the energy spectrum predicted from a blackbody-like source with T≈5×1010K

• Allowing for the efficiency (very, very low!) and the ability to detect only 1 of the 6 neutrino/anti-neutrino types, the observed number of neutrinos agrees with the prediction for the number of neutrinos liberated in the core of SN1987A

• Time of arrival of the neutrinos pre-dated the first optical radiation observed from SN1987A by hours. Why is this?

• The total time over which the neutrinos were observed was 12 s• Can use the timescale for the arrival of the neutrinos to make estimates of important quantities describing interactions of neutrinos and matter at very high densities


SN1987A: Neutrino Detection• Collapse timescale << 1s – not origin of extended time over which neutrinos were detected• Crossing time of the core at velocity of light (R/c) at maximum 40km/300000kms-1 << 1s – not origin of Δt• If neutrinos do not free-stream but possess mean-free-path (<R) then time taken to diffuse out from the core is larger than the crossing time by a factor R/l, where l=mfp of the neutrinos

m1

s102

≈⇒

≈≈∆

l

lcRt ns

• Taking estimate for spread in arrival times combined with idea of neutrinos random-walking out of neutron star with density ρ≈1018kgm-3 obtain direct estimate of mean-free-path

• Neutrino detection from SN1987A a major triumph for supernova core collapse models and physics


Stellar core


Lecture 20: Summary

•Maximum ZAMS mass for formation of a white dwarf can be deduced from observations of young clusters• Supernova types from spectra – Type Ia, no Hydrogen evident, thermonuclear detonation. Core-collapse supernovae for Type II, with Hydrogen and Type Ib and Ic, without Hydrogen•Importance of photodisintegration and electron capture (neutronisation) for collapse of inert Iron cores in massive stars, leading to creation of neutron star and supernova• Energetics require a means to extract majority of gravitational energy released – neutrinos• Observations of neutrino emission from SN1987A provided direct confirmation of model for core collapse and allowed estimate of the mean-free-path of neutrinos at nuclear densities


Picture Credits• Slide 20 and 22 © Phillips, Wiley• Slides 4 © from Ostlie & Carroll, Wesley• Slides 18, 19 and 20 © from CfA Supernova

Group, Harvard• Slide 31 © from Shu, University Science Books

structure and evolution of stars - university of cambridgephewett/sandes2017/lec2017... · 2017....

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